Add kern.features flag for MMCCAM

kern.features.mmcam will be present and equal to 1 if the kernel has been
compiled with option MMCCAM.
This will help sdio-related userland tools to fail-fast if running on the kernel
without MMCCAM enabled.

Approved by:	imp (mentor)
Differential Revision:	https://reviews.freebsd.org/D12386

sys/cam/mmc/mmc_xpt.c

@@ -63,6 +63,8 @@ __FBSDID("$FreeBSD$");
 #include <machine/_inttypes.h>  /* for PRIu64 */
 #include "opt_cam.h"
 
+FEATURE(mmccam, "CAM-based MMC/SD/SDIO stack");
+
 static struct cam_ed * mmc_alloc_device(struct cam_eb *bus,
     struct cam_et *target, lun_id_t lun_id);
 static void mmc_dev_async(u_int32_t async_code, struct cam_eb *bus,